The MRI apparatus irradiates a high frequency wave on a body under examination (hereafter referred to as the subject) placed in a uniform static magnetic field to excite nuclear spins inside the subject, receives MR signals generated by the nuclear spins, and renders an image of the subject (imaging). Irradiation of the high frequency wave and reception of the MR signals are conducted by a resonance type RF coil.
Generally, in imaging by the MRI apparatus, an MR signal of hydrogen nuclide (1H) is used. However, these days, there is multinuclear imaging whereby MR signals from plural kinds of nuclides, such as fluorine (19F), phosphorus (31P), and sodium (23Na), in addition to hydrogen nucleus, are detected and are put into images. The multinuclear imaging is used in the case where an in-body distribution of a component nuclide contained inside the body other than hydrogen is intended to be grasped, in the case where a contrast medium having fluoride is intended to be visualized, and the like.
In the multinuclear imaging, when MR signals from plural kinds of nuclides are detected with a resonance type RF coil, the resonance frequencies of the RF coil must be tuned in to frequencies of the MR signals of the respective nuclides. For example, in the case where an MRI apparatus of 3T imaging two kinds of nuclides, hydrogen and fluoride, the resonance frequency of the RF coil is tuned in to 127.8 MHz for hydrogen and to 120.3 MHz for fluoride, respectively.
For this reason, the multinuclear imaging uses the following two methods: a plurality of RF coils having different resonance frequencies that are tuned in to respective frequencies of the MR signals of a plurality of nuclides that are intended to be detected; and a single RF coil capable of changing the resonance frequency in response to the nuclide to be detected. The RF coil that is used in imaging in which two kinds of nuclides are considered as objects to be detected, which is a main stream in the multinuclear imaging, and that is capable of changing the resonance frequency includes: (1) A coil utilizing a fact that a birdcage type RF coil has a plurality of resonance modes (e.g., see Gerald B. Matson et al., “A Practical Double-Tuned 1H/31 P Quadrature Birdcage Headcoil Optimized for 31P Operation,” Magnetic Resonance in Medicine, pp. 173-182, 1999); (2) A coil changing its resonance frequency using a variable capacitor; (3) A coil changing its resonance frequency by an arrangement of a capacitor and an inductor (a double tunable RF coil); (4) A coil changing its resonance frequency by varying the distance between an RF shield and the RF coil (e.g., see Dongfeng Lu and Peter M. Joseph, “A Technique of Double Resonant Operation of F-19 and H-1 Quadrature Birdcage Coils, ” Abstracts of Society of Magnetic Resonance in Medicine, p. 531, 1990, and JP-A-Hei7(1995)-155307); etc.